Power converters typically include a full wave rectifier responsive to an AC power line voltage. The rectifier drives a series inductor and a switch having an output voltage applied to a storage capacitor network. The switch duty cycle is controlled, frequently at a fixed frequency much higher than the line voltage frequency, so a regulated DC output voltage is developed across the storage capacitor network. To maintain the phases of AC voltage and current components derived from the rectifier as close to unity power factor as possible a controller for the switch duty cycle responds to voltage and current components derived by the rectifier, switch and capacitor. A controller of this type, commercially available from Unitrode in integrated circuit form, is referred to as a High Power Factor Preregulator with the nomenclatures UC1854, UC2854 and UC3854.
The Unitrode High Power Factor Preregulator includes an analog multiplier circuit having an output proportional to ##EQU1## where A, B and C are the amplitudes of analog input voltages applied to the multiplier. In one prior art configuration the values of B and C are respectively proportional to the instantaneous amplitude of the AC voltage component derived from the rectifier and the RMS value of the voltage derived from the rectifier. The value of A is somewhat more complex, being determined by the difference between a reference value and the DC output voltage of the capacitor network and a further factor controlled by abnormal operation of the converter, e.g. overshoot of the DC load voltage following restoration of power to the rectifier after a momentary interrupt of the AC source, lasting for a fraction to several cycles of the power line frequency.
The prior art circuit is referred to as a soft start circuit because it prevents overstressing of the switch and capacitors in the network, as can occur with the overshoot that frequently occurs upon restoration of power after a momentary power line interruption. The prior art soft start circuit has the disadvantage of requiring many cycles of the AC power line frequency to occur before the converter output voltage is restored to a substantial percentage of its desired value. The typical prior art soft start circuit includes a series resistor having a constant value and a capacitor or only a capacitor connected to a constant current source in the High Power Factor Preregulator.
It is, accordingly, an object of the present invention to provide a new and improved power converter with a high power factor preregulator arranged so that the converter output voltage is allowed to be quickly restored to a relatively high percentage of its desired value upon restoration of normal converter operation after abnormal operation thereof.
A further object of the invention is to provide a new and improved power converter with a high power factor preregulator arranged so that the converter output voltage is allowed to quickly restored to a relatively high percentage of its desired value upon restoration of input power after a momentary power loss to the converter.
Another object of the invention is to provide a relatively simple, new and improved high power factor preregulator circuit adapted for use with a power converter, wherein the preregulator includes a soft start circuit arranged to allow the converter output voltage to be returned quickly to a relatively high percentage of desired output voltage upon restoration of normal converter operation and after a momentary loss of same.